1. Field of the Invention
The present disclosure relates to an isolation fitting for a heat exchanger of a heat pump, and, more specifically, to an isolation fitting and a method for electrically isolating the heat exchanger of a heat pump.
2. Description of Related Art
Heat pumps are increasingly replacing fossil fuel heaters, especially in applications where heat pumps are a more cost effective heating method. Air-source heat pumps have been used in various applications to remove heat from the outdoor air and move it to another fluid or heat sink. Applications for such heat pumps include space and water heating, and providing process heat for industrial and commercial applications, including agricultural aquariums, fish ponds, and the like.
While initially overlooked for their higher initial cost, in the last few decades swimming pool heat pumps have become increasingly popular as a more efficient and cost effective alternative to fossil fuel pool heaters, such as natural gas, propane, and oil-fired units. Due to their significant operating cost savings compared to fossil fuel pool heaters, the higher initial cost of heat pumps can be quickly recouped. Such higher cost of heat pumps is largely due to the additional components they have compared to conventional fossil fuel heaters. Typical fossil fuel heaters include a gas control module, a firebox, and a heat exchanger coil. The pool water passes through the inside of the heat exchanger coil, while the hot combustion gases pass over the outside. The coil is usually made from copper. In the event that the coil corrodes or leaks, the entire heat exchanger coil is replaced. Corrosion of the heat exchanger coil usually does no damage to the rest of the heater. While replacement of the coil may be costly and inconvenient, it typically does require replacement of the entire heater.
With reference to FIG. 1, a typical heat pump HP includes an air-moving device 1, such as a fan, and a first heat exchanger 2 adapted for extracting heat from the air moved by the air-moving device 1. The heat pump HP further includes a refrigerant compressor 3 configured for compressing, and thereby heating, a refrigerant gas. The second heat exchanger 4 is adapted for delivering heat from the heated refrigerant to circulating pool water. The second heat exchanger 4 includes an inlet 5 for taking in unheated pool water and an outlet 6 for delivering heated pool water after it passes through the second heat exchanger 4. The inlet 5 and the outlet 6 are connected to a pump and filter circuit (not shown) that circulates pool water through the second heat exchanger 4. A metering device 7, such as an expansion valve, is provided between the first heat exchanger 2 and the second heat exchanger 4. The metering device 7 creates a high pressure on one end thereof for condensing the hot compressed refrigerant gas and a low pressure on the other end thereof to evaporate the cooled liquid refrigerant. The compressor 3 includes a motor that requires high voltage electric power to compress the refrigerant gas against the metering device 7 and circulate it through the first and second heat exchangers 2, 4. The refrigerant is circulated through refrigerant tubing 10, such as copper tubing, that runs between the first heat exchanger 2, the compressor 3, and a second heat exchanger 4.
For safety reasons and to meet various electrical codes, all parts of the heat pump HP which may potentially become energized through a short circuit or other fault must be grounded to a local earth ground. Grounding provides an electrical path for any short circuits and trips a power circuit breaker. The refrigerant lines are typically brazed or silver soldered to provide a seal and a closed circuit containing the refrigerant. Such construction also creates an electrically conductive loop of metal between the heat pump components. In this way, a short circuit to any one component of the heat pump HP will result in a tripped power circuit breaker.
In many wiring circuits, the ground wire can be a significant length away from the power supply box and the local earth ground can build a resistance and create a stray voltage drop should any small leakage of charge occur. Recent trends toward the use of electronic chlorine generators, which use a low voltage and salt to generate sanitizing sodium hypochlorite via electrolysis, create an additional source of stray voltage through the pool water to the second heat exchanger 4. In the case of a swimming pool heat pump, this stray voltage drop becomes critical since the copper tubing and thus the second heat exchanger 4 are in direct contact. This can lead to the formation of a galvanic cell. Since most pool water is treated with chlorine or bromine, or contains some type of salt ions, such pool water is generally conductive and can enhance galvanic corrosion.
Thus, the second heat exchanger 4 of the heat pump HP is subject to high pressure and high temperature due to the flow of pressurized refrigerant therethrough, corrosive chemicals from the pool water, and is subject to galvanic corrosion from any stray voltage. Although the use of ground fault circuit breakers and the adoption of corrosion resistant heat exchanger materials such as titanium have minimized safety and corrosion concerns, they have not eliminated them. Accordingly, it is desirable that the second heat exchanger 4 (or condenser heat exchanger) is electrically isolated from the rest of the heat pump HP to provide additional protection from shock hazard in addition to existing circuit breaker and optional ground fault circuit breaker protection should a short circuit occur between the power supply and the heat pump metal refrigeration circuit tubing. An additional concern with the second heat exchanger 4 is the prevention of galvanic corrosion by isolating the tubes of the second heat exchanger 4 from any stray voltage that can be present between the metal refrigeration circuit tubing and the ground wire from the breaker box. Additionally, if the tubing on the second heat exchanger 4 corrodes and permits swimming pool or spa water to enter the sealed refrigeration system, the entire heat pump HP must be replaced. Therefore, it is also highly desirable to provide a fitting that allows replacement of the second heat exchanger 4 without replacing the entire heat pump HP.
Within the prior art, the Heat Siphon® swimming pool heat pump, manufactured and distributed by United States ThermoAmp, Inc. of Latrobe, Pa., USA, includes a fitting (see FIGS. 1-3) which electrically isolates the heat exchanger from the pool water while maintaining a high pressure seal to contain the hot refrigerant gas and prevent it from leaking out of the closed heat pump piping circuit. Heat Siphon® is a registered trademark (U.S. Pat. No. 3,243,696) of United States ThermoAmp, Inc.
The fitting, shown in general by reference number 8 in FIG. 1 and in detail in FIGS. 2-3, is disposed on the refrigerant tubing 10 between (a) the second heat exchanger 4 and the compressor 3 and (b) between the second heat exchanger 4 and the metering device 7. The fitting 8 includes a first fitting 9 that is joined to a second fitting 12 by a nut 13. The first and second fittings 9, 12 are connected to the tubing 10 of the heat pump HP. The nut 13 is threaded onto external threads 14 of the first fitting 9. A flange 15 of the second fitting 12 is disposed within the nut 13 such that the second fitting 12 may be compressed within the nut 13. The second fitting 12 is typically made from titanium and is welded to the tubing extending to and from the second heat exchanger 4. The first fitting 9 and the nut 13 are generally made from brass and connect to the copper tubing 10 of the heat pump HP. A pair of elastomeric rings 11 is disposed above and below the flange 15 to prevent direct physical contact between the flange 15 and the first fitting 9 or the nut 13. The elastomeric rings are typically made from a Teflon® material or other similar material. “Teflon” is a registered trademark (U.S. Pat. No. 1,592,650) of E.I. Du Pont De Nemours and Co. Corporation, Wilmington, Del., USA. An o-ring 17 is disposed in a groove 16 at the terminal end of the first fitting 9. The o-ring 17 contacts the terminal end of the second fitting 12 as the nut 13 is tightened to provide a seal for the refrigerant flowing through the fitting 8.
While the fitting 8 described above has had a widespread use for a number of years, there are a number of disadvantages associated with this design. Because the second fitting 12 is welded to the tubing of the second heat exchanger 4, the nut 13 and the lower ring 11 disposed between the flange 15 and the nut 13 must be assembled onto the tubing 10 before the second fitting 12 is welded to the tubing 10. In the event that the second fitting 12 is welded without placing the lower ring 11 and the nut 13, the second fitting 12 must be cut off from the tubing such that the lower ring 11 and the nut 13 can be placed onto the tubing 10. This configuration also precludes replacement of the lower ring 11 if it is damaged, physically defective, or if it deteriorates. Additionally, the o-ring 17 must be assembled into the groove 16 with great care such that it does not fall out of position or become pinched as the nut 13 is tightened on the first fitting 9. When installing the o-ring 17 in the field, it is often difficult to maintain it within the groove 16 while assembling the fitting 8.